The present invention relates to a touch sensitive electronic switching circuit including a C-MOS inverter, for use in electronic wristwatches.
Recently, it has been proposed to provide a touch sensitive electronic switch in an electronic wristwatch because the touch sensitive electronic switch is simple in construction and operation as compared with the conventional machanical switch such as a pushbutton switch. Moreover, a switching circuit for the touch sensitive electronic switch can be incorporated in an integrated circuit constituting a time calculation circuit of the electronic wristwatch.
The switching circuit of the touch sensitive electronic switch for use in an electronic wristwatch usually includes a C-MOS inverter. Such a switching circuit is described in copending application Ser. No. 575,731 filed May 8, 1975, by Takehiko Tasaki and Hidetoshi Maeda which is assigned to the same assignee as the present invention. The threshold voltage levels of the C-MOS inverter are unavoidably different from each other due to different makes of the individual C-MOS inverters. When the resistance value caused by the operator who touches the electrodes of the touch sensitive electronic switch is considerably high, there is a possibility that the switching circuit can not change its state. Therefore, it is required, to enhance the reliability of the touch sensitive electronic switching circuit, to shift the threshold voltage level of the C-MOS inverter to a high level or to a low level.